The look and feel of an operating system's user interface provides the primary user experience with the operating system. For example, the user experience is based on what the windows of a graphical windowing environment look like, how they are positioned on the screen relative to other windows, and so forth.
The cost to change the look and feel of an operating system is significant. In general, a design team provides a design, and programmers code the operating system to match what the design team desires. As new innovations and designs emerge, the cost is repeated whenever a new look and feel is specified.